1. Field of the Invention
The present invention relates to a circular polarization illumination device and a plant growth regulation method using the same.
2. Description of the Related Art
It is known that a large difference is exhibited in the growth of plants depending on the quality of light, and for example, the growth is promoted when the plant is cultivated by being irradiated with right-circularly polarized light and is inhibited when it is cultivated by being irradiated with left-circularly polarized light. In order to obtain the circularly polarized light described above, a polarizing plate and a retardation plate need to be used. However, there is a problem that from randomly polarized light, only the light that is half or less of the desired amount of light is obtained in principal.
As the related art, for example, a method of forming right-hand polarized light and left-hand polarized light by a polarization separation element and irradiating light by surrounding one of the light with a shielding plate has been proposed (see JP2008-228688A).
Moreover, when linearly polarized light is irradiated in a certain direction by using an absorptive polarizer, a difference is not exhibited during the irradiation in raising plants (see JP1990-283217A (JP-H02-283217A)).
In addition, a light source as a combination of an LED emitting excitation light and a reflective polarizer has been proposed (see JP2006-515963A). Examples of the reflective polarizer include those containing cholesteric materials, multi-layer polymeric materials, and multi-layer birefringent polymeric materials.
Meanwhile, conventionally, in a plant factory which is a system for planned production of plants in a closed or a semi-closed space having a controlled internal environment, the growth is regulated by adjusting the temperature, fertilizers, time of light irradiation, illuminance, and the like. However, recently, there has been a report regarding a method for promoting growth of plants by means of regulating the quality of light (see JP2007-222039A). Further, a plant growth inhibition method by the irradiation of polarized light has been proposed (see JP1990-283218A (JP-H02-238218A)).